Antenna field pattern measuring system



July 8, 1952 o. H. SCHMITT ET AL 2,602,924

ANTENNA FIELD PATTERN MEASURING SYSTEM Filed Oct. 23, 1947 6 Sheets-Sheet l 2O ACTUAL WINGSPREAD MODEL WINGSPREAD MODEL FREQUENCY ACTUAL FREQUENCY X INVENTORS "7- #0 H. Schmiti 1L9- m'nfieldli; 0mm

Roger E. Avery {Ped'rik Barnes July 8, 1952 o. H. SCHMlTT ET AL 2,602,924

ANTENNA FIELD PATTERN MEASURING SYSTEM Filed Oct. 23, 1947 6 Sheets-Sheet 2 IO Intervals v.5. TEAS.

l5 Intervals- 20 Intervals 30 Intervals a. ll. 48 MB KLYSTRON REMOTE KLYSTRON POWER AND OSCILLATOR '%"L 'Z% CONTROL UNIT (2sso-|o,aoo Mo.)

/4A UHF TRIODE LOW FREQUENCY SCILLATOR f 0-2700 m.) RADMTORS INVENTORS scum: AUDIO WAVE OSCILLATOR QH0 H'SchmLtt W 6P5 mnfleld E Fromm Z Wesiey f1. Fails Roger E. Avery Fl'eolrik R. Barnes July 8, 1952 o. H. SCHMITT ET AL 2,602,924

ANTENNA FIELD PATTERN MEASURING SYSTEM 6 Sheets-Sheet 5 Filed Oct. 23, 1947 /)4 I 46 C W OSCILLATOR I Q (500- |0,3OO Me.) mT o SQUARE WAVE MODULATOR I2 Rom-Tm SERVO AMPLIFIER MODEL 2 222; FosaTnoN INVENTORS STRENGTH A DATA H: I Winfield E. Evmm SREELEEECQEZ REGEWED v sla A. Fails AMPUFIER lioger E. Aver ,Faedz-i% 1 5,. Barnew E I IL/$42M ATT RNEYS y 8, 1952 0. H. SCHMITT ET AL 2,602,924

ANTENNA FIELD PATTERN MEASURING SYSTEM Filed Oct. 25, 1947 6 Sheets-Sheet 4 INVENTORS 0H0 H.Ash zrnzlzl-v A mnfzlelcl E. Fromm Wesley .H. Fails Roger E. Avery BYEtedrik Ewes aw A July 8, 1952 o. H. SCHMITT ET AL.

ANTENNA FIELD PATTERN MEASURING SYSTEM 6 Sheets-Sheet 5 Filed Oct. 25, 1947 S R 0 T E V m 0H0 [i Schmifl Winfield E. Fromm Wesley H. Fails Boyer E. Avery F1 rik R. Barnes @Quh Ar L ATT RNEYS y 1952 o. H. SCHMITT ET AL 2,602,924

ANTENNA FIELD PATTERN MEASURING SYSTEM Filed Oct. 23, 1947 6 SheetsSheeT. 6

MODEL NUMBER 654 FILE No. HZA

MODEL SCALE ///5 PLANE TYPE G MODEL suRFAcEML ANTENNA TYPE MODEL FREQUENMJM ANTENNA LOOATIONM REMARKS Ilnfemm m paraboll'c *1 FULL SCALE FREouENcY befween .sf fi ns 250/2 and 266 SHEET 5 0P6 and I40 49 +0 /70 49 rad/a -i INVENTORS Ulla HSc/zmill m nfz'eld flfiomm ELEVATION ANGLE COORDINATE SYSTEM TrMWE/"S Verf/fa/ Wesley f1. Fouls POLARIZATION E REMARKS Royerflflvew; CURVE PLOTTED IN: VOLTAGE PowERn. DEOIBELS [1. a Barnes @HLU,M+

ATTOR Patented July 8, 1952 -ANTENNA- FIELD P ATTERN .MEASUBH I G YSTEM Otto n; Schmitt, Minneapolis, Minn and .Winfield E. Fromm, East Williston, Wesley A; Fails,

East Hempstead, Fredrik R. Barnes, Westbury,

andRog er-E: Avery, Oyster Bay, N.--Y;.,- assignors to Airborne Instruments Laboratory, Inc,

3 Mineola, N. Y. I4

. 'App lication ctober 2s, 1947,'Sria1No; 7 81 ,616

*9 Claims. 1

This invention relates ;to apparatus 'foriand methods of measuring theoperating characteristics of antennas. More particularly, it relates to apparatus and methods for the automaticand rapid production of accurate operational data of aircraft radio antennas, and ,for recording these data in a conveniently usable form at minimum expense. 1 p y 1 Direct mathematical: calculation of radiation characteristics is sometimes possible and may revealthe general character of the pattern which will be produced byxanew or' proposed antenna structure. Such calculations, 'however, are usually far from rigorous because the precise computation becomes .virtually impossible unless many simplifyingassumptions are made, particularly with respect to boundary conditions.

A skilled designer vprovided with such mathematical estimates, employing the "concepts of optics where they .are'applicable; and aidedlby -a great deal of practical experience in extrapolating from known cases, can often,,make a shrewd estimate of the performance ofanew installation, but such designersare quickto recognize'the fact that ,onlyby actual measurement canreally reliable answers beobtained; Q

In the-frequency rangeswh'ere airplane dimensions are comparable with radio wavelengths, it is particularly diflicult togpredict accurately the radiation pattern which a particular antenna in- "stallation'will exhibit, because thefientire. .air-

plane is actively a part of the antenna system.

However; many factors combine to render, experimentalmeasurements on aircraft antennas very-difiicult. For example, the antennamustbe mounted on the particular typewof aircraftwith which it is to be usedjbecause the characteristics of the aircraft afiect the operation of. the antenna; the airplane'must be aloft so that reflections from the earth -Will not interferewith the measurements; and in'additionthe problem is truly three dimensional and hence requires measurementsover aaiull-jspherical shell. V

Three-approaches to the isolation problemare feasible: -One employs'tests' onfflive airplanes,

one makes use of real or mock-up planes mechanically maneuvered on 2;,highi1313tj0ll1ly3l1d'th6 third is basedon the method of mpdels; In accordance with the present inventionscale models are employed in a system which is accurate, compact, and relatively inexpensive, and which does not require the existenceof an actual airplane before-experimental measurements can be obtained. Moreover, it -yieldsguickly -and in compact usable form, the largebulkof damequired to describea three dimensional ant-enna radiation pattern.

actually will be 'used, sothat the; ratio of" the wavelengthofthetestradioz-slgnalisgf-related to the dimensionsof'the model airplane in the'same manner as the wavelength of the actual'operat- "ing radio signal-isrelated;tofthe dimensions of "thefull size airplanep,

The idea of using scale-models for antenna pattern measurements is not a recent development; It has been'previousfly pmppsedbqthfor airplane and for ordinary antenna measurements in various laboratories: 'Most-pi these-attempts to construct such measuring systems have I, de-

pended, "however, on laborious point by .jpcint measurements. Either the model or the test antennawas moved systematically l 1 some -sort of coordinate system;byihandprbyiemote ntrol. Such a systemds completely impractical begause of the large amount of data required to represent the operatingcharacteristicsoi a singleantenna.

' These data must -represent"the, strength "of radiconsidered asa point source.

the angles of elevation and pazimuth, resin tak n a zq qinli awar irfec P 1}?!. n g e ank grees. These angles; as shaman-rig: Lithus ation which. may; be expected from the; antenna in everydirection l;

' involved in such patterns a; sui table coordinate namllspheriqal. hell. In r o ic ures arlrth amQun pi;dat

system for recording them entsds describedb cw ia-nd -io lowed bya es mategorthe --actual minimum number of measurements requir d n a pa tic laninstan The airp1ane in fiigh t is compare w th he. ex ent!) the .surmund hs. fie

of radio energy so thatif or ,thepurppsesiofgdetermining the; direction 0 arrival; of, radio s pals emanating from.v the aircrait "the: I r'cra'ftmayfbe It s' taken ,;accordingly, as the origi ioi aispherical' polar coordinate fvalues, from'zeroto 9 01 care as seen fioih'aie'pnbrs pos oii ilij th' The angle of elevation, a is' p an pq hehor n 1 nd n a i e mam The a b e the r z n,

we q m i .t anslati n l-enem us is l tion and'azimuth; For'eizampl'e, the direction P position of the aircraft Although it is possible to cover the polar sphere by any systematic variation of these two coordinates; two sets of these have been chosen for regular use. These we call principal plane patterns (as illustrated in Fig. 2) and conical patterns (as illustrated in Fig. 3).

There are three principal plane patterns, horizontal (A in Fig. 2), longitudinal vertical, i. e. vertical fore-and-aft (B in Fig. 2) and transverse vertical, i. e. vertical athwartship (C in Fig. 2). These three patterns, representing three mutually perpendicular plane sections of the sphere can be utilized to give quickly an approximate picture of the whole pattern.

Conical patterns are taken by changing the relative positions of the transmitter and receiver so that 0, the elevation angle, assumes successive values, separated, for example, by 5 or degrees, and the azimuth angle, rotates through 360 at each value of 6. Typical positions of. such coni- 71 cal patterns are shown at D, and'G in Fig. 3.

\ principal plane patterns. Because the horizontal principal plane is included among the conical patterns, a total of2l plane polar coordinate graphs is required to represent the relative strength of the field radiated in the various directions by a particular antenna at. a givenv frequency.

To emphasize the actual amount of data involved in these patterns, consider the number of individual readings required to specify a complete spherical antenna pattern. Fig. 5 shows an actual radiation pattern in a particular plane. From this pattern, point values have been taken around the circle at intervals, respectively of 5, 10, 15, 20, and degrees and utilized for making the antenna pattern curves shown in Figs. 6 to 10,

inclusive. "These points were connected with smooth curves by a draftsman who was not permitted to" see theioriginal pattern. The results establish-that ev'ena 10 degree mesh of point readings is not sufficiently complete if the sharp peaks and nulls are to be apparent. However, assuming that such 10 degree intervals between measurements in each plane are adequate, and

that similar angular spacing exists between the successive elevation angles at which readings are taken, about 500 readings are required to present the entire pattern.

In addition, however, two polarizations are required in order to permit'calculation of absolute directivity, and for this reason the number of measurements is increased .by a factor of two, i. e. 1000 readings are required to determine a single spherical pattern at one frequency. For Wide band antennas this number is further multiplied by the number of frequencies at which the pattern must be repeated. As this number usually does not exceed ten for one antenna, it is apparent that between 1000 and 10,000 measurements are necessary in order to acquire complete pattern information for a single antenna on a particular aircraft.

Recognizing that this large quantity of data is an inherent characteristicof any adequate three dimensional pattern system, it is an object of this invention to provide apparatus and methods for obtaining these data quickly, automatically, and in a readily understandable form, with a minimum expenditure of technical manpower and time.

It is another object of this invention to provide in such a system, a tower for supporting test models and which has improved electrical and mechanical characteristics.

It is another object of this invention to provide apparatus and methods,- for controlling automatically and precisely the various movable components of such a system. I

It is still another object to improve the operation of such a system by providing novel modulation methods and apparatus.

Still another object of this invention is to provide apparatus and methods for recording antenna field intensity patterns and overcoming ambiguities of conventional polarization concepts.

A further object of this invention is to provide such a system in which antenna patterns may be automatically and optionally recorded as a function of voltage or power of the received signal.

Another object is to provide improved radiators for producing a uniformfield over a wide frequency range and in which the angular coverage of the field is to a large extent independent of frequency.

Still another object is to provide an improved mount for such radiators whereby the radiators may be quickly interchanged or rotated to a new position and which is optionally provided with an automatic positioning mechanism and/or a calibrated scale for denotingthe angle of polarization'of the radiated energy.

wIt'is still another object to provide such a system having an extremely low noise level and high stability.

Still a further object is to provide an improved recorder for recording rapidly and automatically the field patterns.

Still another objectis to provide an improved chart for representing graphically the field pattern and improved methods and apparatus for positioning and securing the chart to the recorder.

The invention, accordingly, consists in the features of construction, combinations of elements, arrangement of parts, and methods of operations as will be exemplified in the structures and sequences and series of steps to be hereinafter. indicated-and the scope of the-application which will be set forth in the following claims.

. Although, in accordance with the provisions of the statutes, there is illustrated and described the best form of the invention now known, it will be apparent to those skilled in the art that the system is complex and contains many interrelated components, each of which is subject to changes that can be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that certain features of the invention may be at times used to advantage without a, corresponding use of other features.

A better understanding of the invention will be had from a consideration of the following description of one embodiment of the invention taken in conjunction with the accompanying drawings in which:

' Figs. 1, 2, and. 3 illustrate one coordinate system ticular system; I

Figs. 5, 6, 7, 8, 9, and 10 illustrate the accuracy obtainable with various plotting procedures;

Fig. 11 is a block diagram of certaincomponents of the system;

: .Fig; -12.shows general arrangement of the radii ating' structure'and the movable tower with a model airplane in position thereon;

Fig. 13 is another block diagram showing components of the system;

Fig. 14 is an enlarged perspectiveview of the tower head shown in Fig.

Fig. 15 is an exploded view showing in perspective the various components of the tower head shown in Fig. 12;

Fig. 16 shows an electromagnetic radiator and the mounting arrangement therefor;

Fig. 17 is an enlarged View of -.the. antenna mount shown in Fig. 16; I

Fig. 18 is a perspective view oi-ap'olar recorder for recording automatically the field pattern; and

Fig. 19 shows a chart with a typical field pat.- tern recorded thereon.

In accordance with the present invention, a scale model airplane 2 (Fig. 12) .completewith a corresponding scale model antenna, and .containing a rudimentary receiver, is rotated mechanically in auniform radio field produced by a special parabolic electromagnetic radiator 4." A pattern 6 (Fig. 13) is produced by plotting the signal strength at the receiver inthe model airplane l against the angular positionofthe airplane with respect to the-source of the radio field, i. e. parabolic radiator 4 01 horn';type radiator 43 (Figs. l2'and 13). '1 a The validity of this measuring method, in the case of a receiving antenna, is based directly on classical electromagnetic theory. Where a transmitting antenna is under test, its model is used also as a receiving antenna, in accordance: with the theorem of antenna reciprocity, which states thatan antenna has the same radiation pattern whether used for. reception or transmission.

iA CW transmitter, "generallyindicated-at 8 (Fig. 13), :whichincludes a square. wave modulator I2 operating at an audio-rate; jin'thislexample 577 cycles per second, and a tunable oscillator [4, in this example, adjustable over the range from 500 to 10,300 megacycles, produces radio frequency energy at therequired frequency which is selected in a manner to be described;later.- A highly di et a ten i rexa p rh tv e di t rA r e i h thi en eypred es a nifo m be m a .radia on. e era ly-1 indi t t 5 .d se uslns eral f et w de and. a wf et b sr un r 1 e e rn ane. .2...- n pl te. with ca mo l ;-al e n a and-:rudime tarr ece s m un edQna-l sht-r onv .eiallie tow l t n orm n ra part Qf.thebe m. IQ-: The t wer is is mechanically arranged to rotate the model in sui abl co dina es itqr esentallaspectsrof the model successiyely 5 to h] the ,radiated; :beam. le tric u swh h are aiur ctionn r e-t w r nd mo l PQ it Qu as w l the-r ceiredtsi nal s ren e e're red. be in an. utqmatic .rei-. see er.th h -s13 agd.--2 ten rnnt plott n Q t e g-Fera tre t a a nst;.an ular p sitionettbemq e .pl ne lr latir to he r di: ation source 4. H I Y it; iszd s abl tba 1,t 1e .;m9d ls abegsmallaand l htenoush t r rmitco en enthandlin and storage. They are constructed, ordinarily so that t w ngsr ead does n ie ceed eieetas-an exeme imi and usua ly. don texeeed. 4 :feet in aximu imensi .JI e s re ethdt he tower l8 may e eadnyamadesufi c e titoipermit t segcfmode s .ygei hing up (tn; 5-;nqunds; The sm llest me els. which.; can;be :used l'fia'dilyiiare' between 6 inches and 1 foot in wingspgeadit'fhtse 6 minimum. dimensions rare-desirablebecause of the increasing'delicacy of the model making work and particularly because of the necessity of keeping the plane large relative to its mounting fixtures, in order to minimize the effect of the adjacent dielectric structures; n The model 2, is constructed advantageously from, light, easily worked wood, for example, sugar .pine, spruce, or; mahogany. For. large models the interior .is hollowed out to reduce the weight. The model, when shaped. to the desired configuration, is coated with a conductive covering, for example,-:by, flame-spraying successively with aluminum and copper to makezthe surface highly conductive yet easily solderable without excessive increaseirrweight. 1.

-A typical model maybe made by-a skilled model maker in about one working .week and once made is usually tested with a variety of antennas, one antenna often being moved; from-place to place until a satisfactory location is found. Patches on the model maybe made readily with sheet copper and solder. T l

Fig. 4 shows the frequency range'in which measurements are madeand the limits over which one particular system embodyingthe invention is designed to operate.

Because electrically conductivematerials :near the model airplane 2 would; interfere, with the measurements, the .derrick portion ,ofthe tower I3 is constructed of selected dry:spruce strips-24 with thin mahogany bulkheads 26 and isbraced-with tensioned lacings oi" fiber glass cord, 28, the wood and lacing being impregnated with suitable plastic material. The glass cord 28'has elastic constants comparable withthose-pf piano wire so that the resultant toweris exeremely rigid as well as light and non-reflective. Mechanical power is transferred to the tower head by a dielectric torque tube extending upwardly through the interior of the tower. Inorder to rotate the model, the tower I8 is provided with a tower head 32 (Figs-14 and 15) which containsspiral bevel dielectric gears 34 and 36- and-ball bearings of glass or other suitable --material. -A "draw-in collet, which may be tightened bya control mechanism 38, permits rapid and secure attachment of models to the head.

The tower has three independent powered motions: (1) translation bodily along the trestle, (2) rotation about a vertical axis, and (3) rotation of the model about its mounting axis.-

In order to rotate the airplane model 2 about itself as a center, rather than about the tower I8, the tower is tilted at an angle, for example, 15 from the vertical. This arrangement decreases the width of radio beam required, and at the same time keeps the tower l8 and tower head 32 somewhat out of the field.

The effect of the cable, which returns the signal from the model, on .the radiated field probably constitutes the largest single source of error in the system. .The effect of the signal cable on the pattern is generally small, and maybe ascertained readily, in order to place the cable in the best possible position, by moving the cable to various positions and noting the. effect on the recorded antenna pattern or on a suitable meter.

In order to eliminate the necessity for slip rings or similar arrangements and the difiiculties attendant with their use, the continuous rotations of the tower and, of the model about their respective axes are limited bystops and limit switches, forv example, to.somethingl less than two rev lutions.

In order to protect the tower when not inuse. it is constructed so that it can be lifted from the trestle and rolled into asuitable weatherproof shelter-by means of casters mounted in the bottom of the tower base 42.

All circuits to the tower are carried through a single 44 conductor cable, except for the Signal return cable which is separate and is doubly shielded for better-isolation. This signal cable extends downwardly on the side of the tower 18 away from the model and its slack is taken up by a springetensioned cable reel which thus prevents it from becoming tangled about the tower.

Provision toplug in a soldering iron or other 115 volt device, and a jack for connecting a hand telephone set for communication with the operating position are provided on the tower base 42, as is also a ruggedly encased remote output indicating meter which isuseful in tuning the model.

Dependin upon the location of the antenna under test the model airplane 2 may be mounted with either the under or the top side adjacent the tower,v thus minimizing the effect of the mount by which the airplane is secured to the tower.

The. motion of the toweriB along the track 44 is useful in preliminary adjustment'procedures. By causing thetower to walk across the beam of radiation l6,"it is possible to determine the degree ofuniforrnity of the beam, and to establish its exact center. If the tower is made to move parallel with the direction of the exciting radiation the existence of any standing waves due to reflections from surrounding objects is at once shown by periodic variations in the returned signal in contrast to the normal steady change with distance.

To avoid ambiguity, two new polarizations E and E have been defined. These replace the usual horizontal and vertical polarization con cepts, respectively, but are identical with them at low elevation angles and are not ambiguous at high angles. Eqb is defined as the polarization in which .the electric vector is along the direction of changing (Fig. 1). Similarly, E0 has its electric vector along the direction of changing 9. These two polarizations are orthogonal, and are always perpendicular to the direction of arrival of radiation.

The actual tower used for measurement is linked to the coordinate system described above. The model airplane is always mounted on the tower, as shown in Fig; 12, with its normally vertical axis in a horizontal plane.

To produce the first of the principal plane'patterns, i. e. the horizontal plane pattern A (Fig. 2) the axis of rotation of the model 2 is set perpendicular to the direction of propagation of the incident beam [5 and the model rotated about its mounting axis. 7

By rotating the model about its mountingaxis until the plane is heading directly into the incident radiation .16 and then causing the tower [8 to rotate on .its vertical axis, the longitudinal vertical pattern. 13. isproduced.

Similarly, bysetting the model so as to head vertically either up or down and then "rotating the tower, about its vertical axis, the transverse vertical pattern C is obtained.

A member of itheconical. family of patterns (Fig. 3) is produced whenever themodel is rotated aboutjits imounting axis.' The elevation angle for any one of these patterns is determined 7 by the angular position of the vtowerhead with respect to an imaginary line extending between the model 2 and the source of the radiation 4. If the top of the plane is directly toward the radiation source, the positive degree elevation conical pattern is'produced. If the tower head is pointed 45 degrees away from the radiation source, the positive 45 degree elevation conical pattern is produced. etc.

Each of the three tower motions is actuated by a separate D.-C. servomotor system under control of a single electronicsystem (indicated by block 46 in Fig. 13) which permits operation of any of the drives over a speed range of :1 or more. Speed and direction of motion are selected on. a conveniently accessible control panel, and

exact position of the tower in any of its-three motions is indicated continuously by accurate remote synchro indicators which preferably are directly calibrated.

All primary A.C. power is drawn from a suitably regulated A.-C. voltage which is followed in all D.-'C. circuits by electronic regulators to give a high degree of stability.

. In order to vcover continuouslyv an extremely wide frequency range, for example, from 500 to 10,300 megacycles two separate transmitting oscillators MA and. MB (Fig. 11) are used, each with its own power supply. They are each modulated by modulator [2, which in all cases provides for square-wave modulation with, for examp1e,-50% duty cycle at a precisely controlled frequency, in this case 577 cycles per second has been selected. The stability of this modulator frequency must be good, as will be evident when considering the receiver design.

The lower frequency transmitter [4A which is used, for example, in the range 500-2700 megacycles utilizes a 2C39 oilcan triode in a grid separation coaxial cavity oscillator circuit. The wide tuning range is achieved by utilizing modes of I oscillation; Approximate tuning may be accomplished quickly-by means of prepared tuning charts specifying values of cathode, plate, and coupling adjustment, thus, the frequency may be changed and favorable conditions selected with little loss of time. Final tuning is, of course, accomplished with the use of asuitably'standard ized wavemeter. Transmitter I4A produces ample-energy, for example, two to 50 watts, and usually must be operated below maximum power output to preventexcessivefield strength at the model. I x Y I r The klystron transmitter I4B, which overlaps the above range'and covers, for example from 2660 to'10,-30' 0- megacycles, 'uses very precisely regulated power supplies (48 in Fig. 11) for re-v flector,- grid, and accelerator voltages. These supplies and their controls are housed in the transmitterrack-g at the operating position but the klystron itself is mounted in a small remote unit containing only the klystron, its blower, its R.-F. impedance matching transformer and necessary safety-devices. In this way the klystron can be located close to the transmitting radiator 4 and connected to it by a few inches of coaxial cable, but can still'be controlled from the transmitter once it is tuned. A power output indicator,. actuated from-a crystalprobe in the klystron output lead, provides an output power indication bothat the transmitter: and at the klystron for use intuning.

the present: example; five refiex .klystrons, forexample; of the12K'.typ'e; are required to cover the. entire band and: these are mounted permanently in plug-in unitswhich can. be slipped individually into "the. remote unit. Each. of these klystrons has a total tuning range of about 30% but may'b'erdainage'd by repeated-wide range tuning;' ther'eforeftheklystrons are arranged to overlap infrequency .-'so' that the required tuning range of'each' klystron is restricted; storage space for 8 klystr on plug-in units is provided in the main tuner rack. f

Advantageously; thefield strength is monitored by a crystal probe and is continuouslyrecorded by" means of a 'suitable recording milliammeter. Itis desirableior -this work, to have a beam of radiationthe' angular width of which remains constant and whichis just sufiiciently wide to illuminate themodel uniformly but-not wide enough that excessive energy strikes nearby floors oiffotherobje'cts. It istypical of highly directional broad-band radiators,-h owever, that their beam widths become narrower as the frequency increases. i w 'In accordance with thepresent-invention, a family :of 5 radiators are provided, each covering about one octave, which give nearly uniform angular coverage of 5 7 intheir' ranges, in both H and E planes, and have very little'cross-polarization. The two radiators l for the lower freual ty ranges, for example 500 to 1000, 10 to 1Q0, and 1750 to 31 50fmegacycles' use parabolas v ith defocused ;wideband v exciter's. Two interchangeable exciter s' permit double use of a 72 inch r l we lglai 'i q in bolah a fix edexciter. g 1

Tw m qak ems j re p ovided to cover he ranges 00p 700 anewrg mme s- Each. t e .raqia p fis. f d by a tapered-ridge waveguide exciter which maintains about 2:1' SWR'over the octave to a ohm line. The horn patterns areimproved by-dielectric correctors placed at the mouth of thehorn.

Each of the radiators is adapted to fit into'a mount 52 (Figs. 16 and 17) which is constructed so that they can be readily; interchanged in a fewimoments. The-antenna mount 52 is fitted with ball bearings and advantageously with a circular indexing plate so that] the antenna may be. quickly rotated to provide-either E or E0 polarization. Preferably automatic stops are provided which makeit; unnecessary to realign the radiators after.; initial installation. Cross polarization for these .antennas averages about 3% so that they may be regarded as good sources 1 of plane polarized radiation.

The portion ofthe receivingsystem actually contained in the. airplane model consists .primarily of an R.-F. impedance matching system which may be of. the single or double stub sort and a bolometer which serves as a load. The stubs are tuned by means ofja detachable tuning tool which reaches into the interior of the plane. The bolometer may .be either a specially designed Wollaston wire unit, such as that designed by Sperry,'or it maybe a common instrument Littlefuse, for example of 5 or 10 milliampere rating. The bolometer is constantly heated by D.-C. to a temperature not far below burnout. When modulated radio. frequency energy is applied to thejuse, its wire heats and cools in response to changes, in the power level, and consequently its resistance changes: Such changes in resistance in a circuit carrying current is equivalent to an' audio frequeny- 'voltage in- 10 put at modulation'frequency.;- This audio. signal is'ledby adoubly shieldedicable to -a high gain audio amplifier 54s (Fig. 13).located: in the re,- ceiver rack.

At the higher: frequenciesrit is necessary to operate at very low: levels of radio frequency power because-oithe limited power output of. h klystron transmitter '|4B,I-'fQI, example, .25-1 watt; amplifier 54, therefore, musth'ave very high gain, 154 db in this'example, anda'correspond- 'ingly low noise levelzi ln order to -reducethe noise level the amplifier. -55 is-sharply tuned. and may have, advantageously, abandwidth at 577 cycles ofthe order of14- cycles 1381'" second. This 'per mite relatively rapid-responsewith an equivalent background audio noise 'level, at the bolometer, oi of a microvolt 'or less. which is barely above resistance noise level.

It will lee-remembered that the change inrbolometer resistance is brought about in response to temperature change which in;turn is proportional to power input,- rather than voltage. This is advantageous, if it is desired to record patterns in terms of power, because the-bolometer, amplifier yields this information automatically.

However, if it isdesired, that therecorder out put be in terms of antenna voltage insteadof power, it is necessary 'to plot the square root of the received signal. The extraction of this square root is accomplished in a jspecialsquareroot section of the amplifier 54 through introduction of non-linearfeedback. I

The square root section of the amplifier. has a gain of b u 9 m nals hangs greater gain for smalll s gnalsandlesser gain for large, adjusted in non linearit y Within 2% of square root over a powe'rjrangeof 1000 to one. The square root amplifiercan be switched 'in or out of the circuit as desired. 5 l A The entire high g""n amplifier and the bolometer are fed with rectifiedpower from" single well-regulated power pack. .In order to further increase the stabilityand reduce the 'possibility of oscillation, which might be;occasionedbygthe high gain andtheuse of 'sharply tuned-bank-pass circuits, 2. very low power. supply impedance, e. g. about 5 ohms, is employed together-with. good decoupling and shielding. Alternating current may be used throughout .f 'r; heatingv the i'ilaa ments.

It might be expected] that preamplification would be used in pref e'ranc t o operatinga cable approximately feetlong at aud'io si hal levels below 1 volt, yet this has proved unnecessary. Output of the high gain amplifier is rectified and fed as D.-C. to the recorder.

The recorder 22 tFig'. "18) permitsea'syplotting of any ordinary A -C:. or D.- C. voltageas a radial distance against angles, given; by synch rof or selsyn data. Sinceit is, desirable to have angles accurately represented within a] fraction" of a degree, turntable 56 derives its motive power from a separate servomotorirather' than directly from selsyns). goveind'I' in turn, by a .control transformer type synchro. Q'I-h'e're. is,- i'riI fact, a double system of 'controlavailable .whereby a sensitive relay switches irom one to-ona-Qto 36-to-one synchro operationfor-vernier positioning of the turntable:-: -Angular accuracyis thus preserved to 0.25: degree "OffbBttBI'; Switching equipment connects the turntable servo to any one of the 'tower'axe'schosen; H A recording" pen 458,1; driven 'byj a seco servomotor of exceptionallyllow; inertia, writes on the polar charts in accordance with the signal from the high gain amplifier. The accuracy of the radial trace is assured since the tone position is controlled by a precision potentiometer followup system equipped with rate circuits to assure high speed deadbeat operation.

'In order to permit fast paper loading, the pen arm 62 can be lifted like a phonograph pickup arm and a sheet'of paper or chart form 64 inserted or removed. 7

It is always difficult to mount coordinate paper accurately in a" specified position on a platen since paper shrinks and swells and is seldom accurately dimensioned or punched. In accordance with the present'invention, this mounting is accomplished by providing a pinhole at the exact center of the turntable 56 through which light shines. This light spot is set in the center of the bulls-eye on the paper 64 and a scribed radial line on the turntable surface is made to coincide with the zeroaxis of the polar paper. The paper or chart 64 is clipped to the steel turntable surface by small Alnico rosette magnets 66 thus permitting easy use of any sizepolar paper up to eleven inches in diameter. At it may be desired to record two polarizations or other distinct patterns on one sheet, additional pens 58 are provided which may be dropped into place in a moment. By the use of various colors of ink, the resulting curves can be distinguished easily.

For making aircraft antenna patterns, specially arranged polar graph sheets printed on tracing paper, so that copies can be produced in quantity either by photographic or blueprinting processes, are convenient. The forms, for example, as shown in Fig. l9, may advantageously carry blocks for recording pertinent data.

Each record pattern is rubber-stamped with a small view of an airplane as at 68, to show the plane in which the pattern was taken, and to reveal at a glance the orientation of the pattern with respect to the airplane.

It'is obvious that, although the components, and combinations of the various components, as well as the entire system described herein have been described particularly as related to the measurement of antenna field patterns, separately and in combination they are adapted to have wide utility for many purposes and it is not intended that they be limited to the ultimate functions described herein.

We claim:

1. The method of determining directional radiation or reception characteristics of a translating device includingthe steps of rotating a first translation device to be tested about a first axis and a second axis perpendicular thereto, successively moving said second axis into a plurality of positions, transmitting an energy containing beam between said first'translating device and a second translating device, measuring the energy received by the one of said translating devices utilized as a receiver, and continuously and automatically recording said measurement as a function of the relative instantaneous angle between said translating devices.

2. The method of determining directional radiation characteristics of an antenna including the steps of producing a directed beam of radio frequency energy having at least one transverse area of substantially constant field intensity, rotating within said area the antenna which is to 12 be tested, and measuring. the magnitude of the signal received by'saidantenna as a'function of an instantaneous rangle o'fsaid antenna relative to" a predetermined reference point.

3. The method as' defined in "claim 2 comprising, the fadditionali step" of .'regularly decreasing and increasing the :intensity of said radio frequency field, at rapid;successive, intervals.

4. The methodasadefinedin claim 2 including, the additional step of successively turning said field off and on atrapidly'recurrent regular intervals. A

5. The method asde'fined in claim 2 comprising the additional stepsof modulating said radio frequency. field, and detecting the signal received by said antenna at, a physical location proximate said antenna and-byia detectorrrotating'therewith. p

6. The; method of determining the field pattern of an aircraft antenna when mounted on 'a particular type ,ofaircraft, in which a sealer model of the aircraft and antenna is'utilizedin the measurements, said method comprising the steps of rotating said model about a predetermined axis; radiating a beam of electromagnetic energy having a wavelength related to the wavelength atwhich said antenna is to be used by -the same ratio by'which the physical dimensions of said model arerelated to the. physical dimensions of said aircraft; synchronizing the rotation of a chart with'the rotation of said model; and recording automatically thereon a polarplot representing the relative strength of the signal received by said model antenna as a function of the angle of said model relative to the direction of propagation of said electromagnetic beam.

7. The method of determining directionalralating signal, determining the magnitudeo f said amplified signal, and'automatically recordingit as a function of the instantaneous angle between said antenna and a predetermined reference point.

8. An antenna pattern'measuring system comprising .in combination, a variable frequency signal generator, a squarewave modulator for producing a pulsating signal from said generator, a directional radiator connected to said signal generator for producing a radiationbeam having therein an areaof substantially uniform illumination, means for rotating the antenna to be tested in said area, and means synchronized with the rotation of said antenna for recording automatically the. strength of the signal received by said antenna.

9. A model aircraftfor use in determining antenna field patterns comprising a wooden body portion, a substantially continuous coating of aluminum 1 or similar material over said body portion, an outer covering of copper over said aluminum, anantenna mounted on said aircraft, and a bolometertype detector connected to said antenna and situated within the body of said aircraft.

REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS Number Name Date Darbord Aug. 22, 1933 Number 14 Name Date Taylor Feb. 19, 1935 Dziewior Mar. 11, 1941 Vrooman June 17, 1941 Busignies et a1 Nov. 18, 1941 Blumlein Jan. 6, 1942 Johnske et a1 Apr. 14,1942 Sendretto et a1. Aug. 27, 19 6 Godet Dec. 17, 1946 Olden May 6, 1947 Iams Apr. 18, 1950 Worthington Apr. 10, 1951 

